1. Field of the Invention
The present invention relates to audio signal processing and in particular to a method and apparatus for adjusting the relative amplitudes of different frequency bands in an audio signal while minimizing undesirable changes in tonal qualities of the audio signal.
2. Description of Related Art
One known method of audio signal processing is to process the audio signal in the frequency spectrum by separating the audio signal into multiple audio signals that fall within spaced audio frequency bands. This separation of the audio spectrum into audio frequency bands can be achieved with analog or digital techniques. The audio signals having frequencies that fall within each frequency band are processed using an audio processing technique and the processed signals are then combined to form a final processed audio output signal.
In the most common form of this multi-band audio signal processing method the amplitude of the signal in each audio frequency band is varied by a fixed or adjustable amount that is independent of the signal within each audio frequency band. The amplitude change in a single audio frequency band will be referred to herein as a “gain adjustment factor.” The gain adjustment factor may be independent of the signals within the audio band, or it may vary and be the result of applying a specific audio processing technique to a varying audio signal within the audio band.
When the gain adjustment factors are independent of each other and of the input signal, this type of audio signal processing method can be considered to be a general-purpose audio filter. The gain adjustment factors may be externally provided and selected to produce a low pass audio filter, a high pass filter, a band pass filter, a band stop filter or other known type of audio filter. A device having this type independent and relatively static adjustment of the gain for different audio frequency bands may be referred to as an audio equalizer.
Audio signal processing methods of this type typically use a reduction of signal amplitude during signal processing to adjust the relative signal amplitudes. The term “gain adjustment factor” is used herein to indicate that signal amplitude within an audio frequency band may be decreased or increased.
In the general purpose audio filter processing method described above, the audio signal in each frequency band is simply changed in amplitude by an externally provided gain adjustment factor. In a related known type of audio signal processing method, the gain adjustment factors are not static and are not independent of the audio signal being processed. The audio signal in each audio frequency band is processed with a more complex audio processing technique such as by applying audio compression, signal amplitude limiting or another known type of audio signal processing. This type of audio signal processing produces an associated gain adjustment factor for each frequency band that dynamically changes depending upon the audio signal within that band as well as upon the specific audio processing technique applied to the audio signal in that band.
In both the simple static design and the dynamic design, digital and analog processing techniques may be used. In each of these known processing techniques, however, the gain adjustment factor for each audio frequency band is independent of the gain adjustment factor in the other audio frequency bands.
One defect in all such audio signal processing techniques is that they tend to produce undesirable changes in the timbre or tone of musical instruments. When an instrument plays a note with a fundamental frequency F, the sound produced will include additional harmonic frequencies at twice the fundamental, (i.e. at frequency=2F−referred to as the “second harmonic” of the fundamental frequency) and at three times the fundamental (frequency=3F−referred to as the “third harmonic”), etc.
The tonal quality and timbre of the sound produced by an instrument when a note is struck is dependent, in large part, upon the amplitude relationships between the fundamental and harmonic frequencies. When an audio signal is processed by one of the known methods described above, the amplitudes of the audio signals in different audio frequency bands are independently changed. The amplitude relationships between the fundamental and the harmonic frequencies are changed whenever the fundamental and harmonic frequencies fall into different frequency bands. This can produce a highly undesirable change in the quality of the sound of an instrument or a singer's voice, etc.
For example, when an audio frequency band of containing frequency f is processed, that band will include not only the fundamental frequency f for instruments playing a note of frequency f, but also the second harmonic of instruments playing a note of frequency f/2 and the third harmonic of instruments playing a note of frequency f/3.
In conventional devices that operate according to the method described above, a relatively small number of frequency bands are used. In devices that separate the audio signal into a relatively small number of bands (three to eight is common) it is likely that adjacent harmonics will be in the same band. This results in the same gain adjustment factor for the fundamental and the close harmonics, reducing undesirable changes in timbre, tone and sound quality.
However, there is a need for audio signal processors that can provide more fine-grained control of the audio spectrum. As the number of frequency bands is increased, the width of each frequency band is narrowed. Each harmonic frequency then falls into a different audio frequency band from the fundamental and will be adjusted in amplitude by a different corresponding gain adjustment factor. As the relationships between the fundamental and its harmonic frequencies is changed by the different independent gain adjustment factors, the tone and timbre of instruments and music is adversely affected.
In this application, the term “primary band frequency” will be used to refer to a frequency within an audio frequency band that characterizes that audio frequency band. It may be the center frequency of an analog bandpass filter, or it may be a frequency defined in a digital processing technique used to separate the audio frequency bands for digital processing, etc.
A primary band frequency of a first audio frequency band may be a fundamental frequency with respect to the primary band frequency of a second audio frequency band. The higher frequency second primary band frequency would be a harmonic of the lower first primary band frequency. At the same time the first primary band frequency may also be a harmonic of a third primary band frequency. The third primary band frequency would be at a lower frequency than the first or second and would be a fundamental frequency with respect to the first.
By way of example, the first primary band frequency might be located at 1000 Hz, the second might be located above the first at 2000 Hz and the third might be locate below the first at 500 Hz. The second primary band frequency at 2000 Hz is the second harmonic of the first primary band frequency located at 1000 Hz. The first primary band frequency at 1000 Hz is the second harmonic of the third primary band frequency at 500 Hz.